U.S. patent number 6,104,486 [Application Number 08/774,272] was granted by the patent office on 2000-08-15 for fabrication process of a semiconductor device using ellipsometry.
This patent grant is currently assigned to Fujitsu Limited. Invention is credited to Hiroshi Arimoto.
United States Patent |
6,104,486 |
Arimoto |
August 15, 2000 |
**Please see images for:
( Reexamination Certificate ) ** |
Fabrication process of a semiconductor device using
ellipsometry
Abstract
A method of fabricating a semiconductor device includes the
steps of illuminating a structure formed on a surface of a
substrate by an incident optical beam incident to the structure
with a predetermined incident angle with respect to the surface,
measuring a polarization state of an exiting optical beam exiting
from the structure in response to an illumination of the structure
by the incident optical beam, and evaluating a size of the
structure in a direction parallel to the surface from the
polarization state of the exiting optical beam, and adjusting a
parameter of production of a semiconductor device in response to
the size.
Inventors: |
Arimoto; Hiroshi (Kawasaki,
JP) |
Assignee: |
Fujitsu Limited (Kawasaki,
JP)
|
Family
ID: |
18365300 |
Appl.
No.: |
08/774,272 |
Filed: |
December 27, 1996 |
Foreign Application Priority Data
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Dec 28, 1995 [JP] |
|
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7-343924 |
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Current U.S.
Class: |
356/300;
250/492.1; 250/492.2; 250/492.22; 250/492.3; 356/128; 356/305;
356/310; 356/322; 356/337; 356/340; 356/364; 356/492 |
Current CPC
Class: |
G01J
4/00 (20130101); G01B 11/02 (20130101) |
Current International
Class: |
G01B
11/02 (20060101); G01J 4/00 (20060101); G01J
003/00 () |
Field of
Search: |
;356/300,305,310,322,337,340,345,351,364,128
;250/492.1,492.2,492.22,492.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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57-132039 |
|
Aug 1982 |
|
JP |
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58-206120 |
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Dec 1983 |
|
JP |
|
61-4905 |
|
Jan 1986 |
|
JP |
|
Other References
"Ultraviolet-visible ellipsometry for process control during the
etching of submicrometer features", N. Blayo et al., AT&T Bell
Laboratories, Murray Hill, New Jersey (Sep. 28, 1994); Optical
Society of America, vol. 12, No. 3/Mar. 1995, pp. 591-599..
|
Primary Examiner: Chin; Christopher L.
Assistant Examiner: Nguyen; Bao-Thuy L.
Attorney, Agent or Firm: Armstrong, Westerman, Hattori,
McLeland & Naughton
Claims
What is claimed is:
1. A method of measuring a size of a structure formed on a surface
of a substrate, comprising the steps of:
storing in a database data representing a relationship between
lateral size and polarization;
illuminating said structure by an incident optical beam incident to
said structure with a predetermined angle with respect to said
surface;
measuring a polarization state of an exiting optical beam from said
structure in response to an illumination of said structure by said
incident optical beam; and
evaluating a lateral size of said structure from said polarization
state of said exiting optical beam according to said data stored in
said database.
2. A method as claimed in claim 1, wherein said polarization state
is represented in terms of a rotation of a polarization plane and
an ellipticity of said exiting optical beam, and wherein said
measuring step is carried out by an ellipsometer.
3. A method as claimed in claim 1, wherein said illuminating step
is carried out for different incident angles of said incident
optical beam, and wherein said evaluating step includes a step of
evaluating an angle of a side wall of said structure with respect
to said surface of said substrate from a combination of said
polarization state and said incident angle.
4. A method as claimed in claim 1, wherein said measuring step is
carried out by using a reflection beam of said incident optical
beam as said exiting optical beam.
5. A method as claimed in claim 1, wherein said measuring step is
carried out by using a diffraction optical beam of said incident
optical beam diffracted by said structure as said exiting optical
beam.
6. A method as claimed in claim 1, wherein said measuring step is
carried out by using a scattered light of said incident optical
beam scattered by said structure as said exiting optical beam.
7. A method as claimed in claim 1, wherein said evaluating step
includes the substeps of measuring a thickness of said structure,
selecting a database corresponding to said measured thickness, and
evaluating one of a width and a cross-sectional shape of said
structure by referring to said selected database.
8. A method as claimed in claim 1, wherein said illuminating step
includes a step of switching said incident optical beam on and off,
and wherein said evaluating step measures said polarization state
of said exiting optical beam for each of a state in which said
incident optical beam is turned on and a state in which said
incident optical beam is turned off.
9. A method for controlling a quality of a semiconductor device,
comprising the steps of a semiconductor device, comprising the
steps of:
storing in a database data representing a relationship between
lateral size and polarization;
illuminating a structure formed on a surface of a substrate by an
incident optical beam incident to said structure with a
predetermined angle with respect to said surface;
measuring a polarization state of an exiting optical beam exiting
from said structure in response to an illumination of said
structure by said incident optical beam;
evaluating a lateral size of said structure from said polarization
state of said exiting optical beam according to said data stored in
said database; and
adjusting a parameter of production of a semiconductor device in
response to said size.
10. A method as claimed in claim 1, wherein said database stores
said relationship in the form of curves.
11. A method as claimed in claim 9, wherein said database stores
said relationship in the form of curves.
Description
BACKGROUND OF THE INVENTION
The present invention generally relates to fabrication of
semiconductor devices and more particularly to a measurement of
profile of a semiconductor structure formed on a wafer during a
fabrication process of the semiconductor device.
In a production line of semiconductor devices, it is necessary to
examine the size of the structures such as agate pattern formed on
a wafer quickly, without contacting to or destroying the structure,
such that the result of the measurement is fed back immediately to
the production line for optimizing various parameters of the
production line. Particularly, there is a demand for an exact
control for gate lengths, as a gate length provides a profound
effect on the threshold characteristic of the semiconductor device
that uses a gate structure.
Conventionally, a wafer carrying thereon a structure such as a gate
pattern is subjected to a scanning process conducted under a
scanning electron-microscope (SEM) for evaluation of the size of
the structure. Further, there is a known process for evaluating the
gate pattern size by measuring a resistance of the wafer by a
bridge circuit that is formed commonly to the gate pattern on the
same wafer.
However, the examination process that uses a SEM takes a
substantial time due to the need of transporting each wafer on the
production line consecutively to a vacuum chamber of the SEM. Thus,
it is not practical to apply such an SEM process to all of the
wafers on the line. Further, even when the SEM process is applied
only to selected wafers, a decrease of throughput is inevitable for
the production of the semiconductor devices. In the case of recent
miniaturized semiconductor devices having a gate length, or other
structural parameters, of 0.1 .mu.m or less, in particular, the
foregoing SEM process tends to cause an error in the result of the
measurement due to the finite or non-infinitesimal diameter of the
focused electron beam used in the SEM as compared with the size of
the structure, wherein the magnitude of the error can reach as much
as 10 nm.
In the case of measuring the resistance by using the bridge
circuit, on the other hand, the result of the measurement cannot be
obtained until the fabrication of the semiconductor devices on the
wafer is completed, although the problem pertinent to the case of
using a SEM such as the poor accuracy or reproducibility of the
measurement may be resolved successfully. Thus, the process cannot
be used for an in-situ feedback control of the production line.
Meanwhile, the art of ellipsometry has been used in the fabrication
of semiconductor devices for measurement of thickness of
semiconductor films and insulation films. Further, the ellipsometry
is used also for controlling an etching process at the time of
formation of line-and-space patterns (Blayo, N., et al.,
"Ultraviolet-visible ellipsometry for process control during the
etching of submicrometer features,", J. Opt. Soc. Am., A, vol. 12,
no. 3, 1995, pp. 591-599).
FIGS. 1A and 1B show the construction of an ellipsometer using
conventionally for ellipsometry, wherein FIG. 1A shows a
rotary-photometry type apparatus while FIG. 1B shows an
extinction-photometry type apparatus.
Referring to FIG. 1A, the ellipsometer includes an optical source 1
for emitting an optical beam, wherein the optical beam emitted from
the optical source 1 is converted to a linearly polarized beam
having a predetermined plane of polarization and the linearly
polarized beam thus formed hits a specimen 3 on which a film to be
measured is formed. After reflection by the specimen 3, the
linearly polarized beam is converted to an elliptically polarized
beam characterized by an angle .o slashed. indicating the direction
of the major axis of the ellipse and an ellipticity k defined as
k=a.sub.min /a.sub.max as indicated in FIG. 2, wherein the
parameters .o slashed. and k are related to ellipsometric
parameters .psi. and .DELTA. to be used later in the description
according to the relationship
where there holds a relationship of
In other words, it is possible to convert the set of the parameters
(k, .o slashed.) obtained by the photometry to the parameters
(.psi., .DELTA.). It should be noted that the parameter .DELTA.
represents the phase shift of the optical beam.
The elliptically polarized beam thus formed is then detected by a
detector 5 after passing through a rotatable analyzer 4, wherein
the ellipsometer of FIG. 1A carries out the detection of the
intensity of the optical beam reaching the detector 5 while
rotating the analyzer 4. Further, a quarter-wavelength plate 4a,
which induces a phase shift of a one-quarter of wavelength in the
optical beam passing therethrough, may be inserted between the
analyzer 4 and the specimen 3 as necessary.
In the ellipsometer of FIG. 1B, on the other hand, a rotary
quarter-wavelength plate 4b is inserted between the rotary analyzer
4 and the specimen 3, and the elliptically polarized beam reflected
by the specimen 3 is converted once to a linearly polarized beam.
The rotary analyzer 4 is thereby rotated in search of the
extinction angle in which the optical beam reaching the detector 5
is interrupted.
As explained previously, the ellipsometer of FIGS. 1A or 1B has
been used successfully for the measurement of film thickness in the
fabrication process of semiconductor devices. On the other hand, it
should be noted that such conventional ellipsometry has hitherto
discarded the information about the lateral size of the structure,
which the polarized optical beam has inherently picked up when
passing through the structure laterally. In conventional
ellipsometry, there has been no proposal to make use of the
ellipsometry for measuring the lateral size of the structure such
as a line-and-space pattern formed on the specimen based upon the
polarization state of the optical beam passed through the structure
laterally.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to
provide a novel and useful fabrication process of a semiconductor
device wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a fabrication
process of a semiconductor device that uses ellipsometry for
measuring the size of a structure formed on a wafer efficiently
with high precision, without destroying or causing a damage to the
structure.
Another object of the present invention is to provide a method of
fabricating a semiconductor device, comprising the steps of:
illuminating a structure formed on a surface of a substrate by an
incident optical beam incident to said structure with a
predetermined angle with respect to said surface;
measuring a polarization state of an exiting optical beam exiting
from said structure in response to an illumination of said
structure by said incident optical beam; and
evaluating a size of said structure in a direction parallel to said
surface from said polarization state of said exiting optical
beam;
said method further including a step of adjusting a parameter of
production of a semiconductor device in response to said size.
Another object of the present invention is to provide a method of
measuring a size of a structure formed on a surface of a substrate,
comprising the steps of:
illuminating said structure by an incident optical beam incident to
said structure with a predetermined angle with respect to said
surface;
measuring a polarization state of an exiting optical beam exiting
from said structure in response to an illumination of said
structure by said incident optical beam; and
evaluating a size of said structure in a direction parallel to said
surface from said polarization state of said exiting optical
beam.
Another object of the present invention is to provide a method for
controlling a quality of a semiconductor device, comprising the
steps of:
illuminating a structure formed on a surface of a substrate by an
incident optical beam incident to said structure with a
predetermined angle with respect to said surface;
measuring a polarization state of an exiting optical beam exiting
from said structure in response to an illumination of said
structure by said incident optical beam; and
evaluating a size of said structure in a direction parallel to said
surface from said polarization state of said exiting optical
beam.
According to the present invention, it becomes possible to measure
the size of the pattern formed on a wafer easily in a short time
during the fabrication process of a semiconductor device, without
contacting to or without damaging the fabricated device. Further,
the result of the measurement can be fed back immediately and in
real time to the production line for controlling the quality of the
semiconductor devices produced by the production line.
Other objects and further features of the present invention will
become apparent from the following detailed description when read
in conjunction with the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B are diagrams showing the construction of a
conventional ellipsometer;
FIG. 2 is a diagram explaining the elliptical polarization measured
by the ellipsometer of FIGS. 1A or 1B.
FIG. 3 is a diagram showing the principle of the present
invention;
FIG. 4 is a diagram showing a part of FIG. 3 in an enlarged
scale;
FIG. 5 is a diagram showing a first embodiment of the present
invention;
FIGS. 6A and 6B are diagrams showing a line-and-space pattern to
which an ellipsometry of FIG. 5 is applied;
FIG. 7 is a diagram showing an example of a database used in the
ellipsometry of FIG. 5;
FIG. 8 is a diagram showing a positioning of a wafer in the
construction of FIG. 5;
FIG. 9 is a diagram showing an ellipsometry according to a second
embodiment of the present invention;
FIG. 10 is a diagram showing an example of using a reflection beam
and a first-order diffraction beam in the ellipsometry of FIG.
9;
FIG. 11 is a diagram showing a production line of a semiconductor
device in which the ellipsometry of FIG. 5 is used according to a
third embodiment of the present invention;
FIG. 12 is a diagram showing the principle for calculating the
ellipsometric parameters .psi. and .DELTA. in the ellipsometry of
FIG. 11;
FIG. 13 is another diagram showing the principle for calculating
the ellipsometric parameters .psi. and .DELTA. in the ellipsometry
of FIG. 11;
FIG. 14 is a flowchart showing the process conducted in the system
of FIG. 11;
FIG. 15 is a diagram showing the relationship between the
ellipsometric parameters .psi. and .DELTA. and a pattern width W
obtained in the ellipsometry of FIG. 5 for a line-and-space pattern
formed by an etching process;
FIG. 16 is a diagram showing the change of the ellipsometric
parameters .psi. and .DELTA. with the progress of etching observed
by the ellipsometry of FIG. 5;
FIG. 17 is a diagram explaining the principle of a fourth
embodiment of the present invention;
FIG. 18 is a diagram showing the details of the ellipsometry of
FIG. 17;
FIG. 19 is a diagram showing an example of a characteristic curve
obtained by the ellipsometry of FIG. 18;
FIG. 20 is a diagram showing the effect of pattern cross-sectional
shape on the ellipsometric parameters .psi. and .DELTA. according
to a fifth embodiment of the present invention;
FIGS. 21A-21C are diagrams showing various cross sectional shapes
of the line-and-space patterns corresponding to FIG. 20;
FIG. 22 is a diagram showing the effect of pattern thickness on the
ellipsometric parameters .psi. and .DELTA. according to a sixth
embodiment of the present invention; and
FIG. 23 is a flowchart showing a process of pattern evaluation
according to the sixth embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Principle
First, the principle of the present invention will be described
with reference to FIG. 3 showing the process for obtaining a width
W of a pattern 11 formed on a substrate 10 based upon a
polarization state of an optical beam incident to the pattern
11.
Referring to FIG. 3, it should be noted that the incident optical
beam enters the pattern 11 with an incident angle .theta., wherein
the optical beam thus entering the pattern 11 is a linearly
polarized beam having a polarizing angle .o slashed. and
characterized by electric field components Ep and Es crossing with
each other perpendicularly.
When the incident optical beam has crossed the pattern 11 from a
first side thereof to a second opposite side thereof, the electric
field components Ep and Es are changed respectively to Ep' and Es'
due to the difference of refractive index and reflectance between
the pattern 11 and the substrate 10 or between the pattern 11 and
the air and further due to the phase difference caused in the
optical beam as it passes through the pattern 11. The electric
field components Ep' and Es' form an elliptically polarized
beam.
FIG. 4 shows a part of FIG. 3 in an enlarged scale.
Referring to FIG. 4, the pattern 11 includes pattern elements 11a
and 11b, and linearly polarized optical beams a-c entering the
pattern 11a exit therefrom as elliptically polarized optical beams
after experiencing refraction and/or reflection.
For example, a monochromatic linearly polarized optical beam having
a polarization angle of 45.degree. and incident to the pattern of
FIG. 4 with an incident angle .theta. forms an elliptically
polarized optical beam after reflection, due to the change of
reflectance at the interface between the pattern 11a and the
underlying substrate 10 for each of the p- and s- components Ep and
Es as well as due to the phase difference caused in the optical
beam as it passes through the pattern 11a. In such a case, it is
possible to obtain a complex reflection coefficient ratio Rp/Rs by
means of an ellipsometer, wherein the foregoing ration Rp/Rs is
determined by the factors such as the refractive index of the
substrate 10, the refractive index of the pattern 11, the shape of
the pattern 11 and the incident angle .theta. of the incident
optical beam.
The ellipsometry using an ellipsometer provides parameters .psi.
and .DELTA. as a result of the ellipsometric observation, wherein
the parameters .psi. and .DELTA. are related to the complex
reflection coefficient ratio Rp/Rs according to the relationship
##EQU1## where the parameter .psi. is defined as
tan.psi.=.rho..sub.p /.rho..sub.s as indicated in FIG. 2, while
.DELTA. represents the phase difference. Both of the parameters
.psi. and .DELTA. are the quantities observable by the ellipsometry
as noted previously. In EQ. (1), it should be noted that the
summation is taken for all of the rays that pass through the
pattern 11.
Summarizing above, the parameters .psi. and .DELTA. are obtained
from the elliptic polarization state of the exit optical beam that
includes the components Ep' and Es' by means of ellipsometry,
provided that the refractive index or dielectric constant of the
substrate 10 as well as the refractive index or dielectric constant
of the pattern 11 are specified and the incident angle of the
incident optical beam is specified.
As will be seen in FIG. 4, the exit optical beam picks up the
information bout the lateral size W of the pattern 11a in the form
of phase difference, as the optical beam passes through the pattern
11a. Thus, it becomes possible to estimate the foregoing size W of
the pattern 11a based upon the parameters .psi. and .DELTA.
obtained from the polarization state of the exit optical beam. In
an embodiment of the present invention to be described later, a
number of sets of the parameters .psi. and .DELTA. are stored in a
database in correspondence to various patterns of which size and
shape are known for example by SEM observation. Thereby, the
pattern size W is easily obtained by referring to the database
based upon the parameters .psi. and .DELTA. observed by the
ellipsometry.
FIRST EMBODIMENT
FIG. 5 shows the construction of an ellipsometric apparatus sued
for an ellipsometric size measurement of a pattern formed on a
substrate according to a first embodiment of the present
invention.
Referring to FIG. 5, a substrate 22 carrying a pattern thereof is
mounted upon a stage 21 of the apparatus, and the substrate 22 is
illuminated by an optical beam produced by a He-Ne laser 23. The
optical beam thus produced has a wavelength of 6328 .ANG. and is
passed through a polarizer 25 such that the optical beam is
converted to a linearly polarized optical beam having a
predetermined polarization angle, which may be set to 45.degree..
The linearly polarized optical beam thus produced then impinges
upon the substrate 22 on the stage 21 with a predetermined incident
angle such as 70.degree.. Alternatively, it is possible to dispose
the quarter-wavelength plate 24 between the substrate 22 and the
polarizer 25.
The linearly polarized optical beam thus impinged upon the
substrate 22 is then reflected after passing through a pattern
formed on the substrate 22, wherein the optical beam, initially
being a linearly polarized optical beam, is converted to an
elliptically polarized optical beam. The elliptically polarized
optical beam thus formed is further passed successively through a
quarter-wavelength plate 24 and a rotary analyzer 26, wherein the
rotary analyzer 26 converts the elliptically polarized optical beam
again to a linearly polarized optical beam, and the linearly
polarized optical beam thus obtained is detected by a photocell 27.
The detection by the photocell 27 is carried out while rotating the
analyzer 26.
The photocell 27 produces an output signal corresponding to the
intensity of the optical beam incident thereto and supplies the
output signal to a processing apparatus 30 after amplification by
an amplifier 28 and an analog-to-digital conversion by an A/D
converter 29. The processing apparatus 30 thereby obtains the
polarization state of the optical beam incident to the photocell 27
and produces the foregoing parameters .psi. and .DELTA..
FIGS. 6A and 6B show an example of a resist pattern 22a formed on
the substrate respectively in a perspective view and a
cross-sectional view. The pattern 22a may be any suitable device
pattern formed on the substrate or a line-and-space pattern formed
on the substrate 22 at a suitable location such as a scribe
line.
Referring to FIGS. 6A and 6B, the substrate 22 includes a silicon
substrate 22.sub.1 on which a silicon oxide film 22.sub.2 and a
polysilicon film 22.sub.3 are deposited successively with
respective thicknesses of 100 nm and 153 nm, and the foregoing
resist pattern 22a is formed on the polysilicon film 22.sub.2.
Various resist materials can be used for the resist pattern 22a
such as the CMS resist (trade name) for electron beam exposure,
which is available from Toyo Soda Manufacturing Company, Japan.
The resist pattern 22a may be a line-and-space pattern in which a
plurality of mutually parallel pattern elements 22b are repeated
with a predetermined pitch such as 0.3 .mu.m and is formed on a
suitable location of the substrate 22 with a size of generally 1
mm.times.1 mm. Each of the pattern elements 22b has a height of 150
nm as a result of patterning of a resist film having a thickness of
150 nm. In such a line-and-space pattern, it should be noted that,
while the pattern pitch can be controlled exactly, the width of the
individual pattern elements 22b may vary variously. When such a
line-and-space pattern is used for the gate electrode of a MOS
transistor, such a variation of the pattern width can cause an
unwanted deviation of the threshold voltage of the MOS transistor
to be formed.
FIG. 7 shows the relationship between the parameters .psi. and
.DELTA. obtained experimentally for the case in which the apparatus
of FIG. 5 is applied to a line-and-space pattern while changing the
pattern width W variously. In the experiment, the substrate shown
in FIGS. 6A in 6B is used for the substrate 22, and the parameters
.psi. and .DELTA. were obtained while changing the width W of the
pattern element 22b from 100 nm to 190 nm. It should be noted that
the value of the width W shown in FIG. 7 is obtained by directly
observing the line-and-space pattern by a SEM. The parameters .psi.
and .DELTA. were obtained for each pattern width W five times.
As will be seen from FIG. 7, there is a one-to-one correspondence
between each set of the parameters .psi. and .DELTA. and the width
W. This means that it is possible to estimate the value of the
width W of the line-and-space pattern on the substrate by measuring
the parameters .psi. and .DELTA. by referring to the relationship
of FIG. 7. Thus, the relationship of FIG. 7 is stored in a database
provided in the processing apparatus 30 of FIG. 5, and the
processing apparatus 30 refers to the database for the evaluation
of the width W.
FIG. 8 shows the apparatus of FIG. 5 in a plan view.
Referring to FIG. 8, the stage 21 carries a positioning member 21A
for engagement with an orientation flat 22A formed on a wafer that
forms the substrate 22 as well as a positioning pin 21B for
engagement with a side wall of the wafer 22.
In the state of FIG. 8 in which the orientation flat 22A engages
the positioning member 21A and the side wall of the substrate 22
engages the positioning pin 21B on the stage 21, it should be noted
that the pattern elements 22b forming line-and-space pattern 22a on
the substrate 22 extend generally perpendicularly to the path of
the optical beam traveling from the laser 23 when viewed in a
direction perpendicular to the substrate 22. By disposing the
line-and-space pattern 22a in such a direction, it should be noted
that the optical beam efficiently picks up the information of the
line-and-space pattern in the form of the phase of the optical
beam. In other words, the sensitivity of detection of the pattern
width W becomes maximum for the line-and-space pattern 22a by
disposing the substrate 22 on the stage 21 in the orientation
indicated in FIG. 8.
It should be noted that the foregoing alignment of the wafer for
proper orientation can be made by various other means. For example,
the wafer 21 may be formed with a cut with a predetermined
orientation such that the cut engages with a positioning pin
provided on the stage 21 when the wafer
22 is placed on the stage 21 with a proper orientation.
Alternatively, it is possible to use an orientation detection
mechanism that uses an LED (light emitting diode) for the detection
of the wafer orientation.
It is of course possible to dispose the wafer 22 in the
construction of FIG. 8 such that the parallel pattern elements of
the pattern 22a extend generally parallel to the path of the
optical beam. By disposing the wafer 22 as such, it is possible to
obtain the information about the wafer underneath the pattern 22a
is obtained.
SECOND EMBODIMENT
FIG. 9 shows a second embodiment of the present invention, wherein
those parts described previously with reference to preceding
drawings are designated by the same reference numerals and the
description thereof will be omitted.
Referring to FIG. 9, it will be noted that the pattern elements 22b
are formed relatively sparsely on the substrate 22. Thus, the
optical beam emitted from the optical source 23 tends to pickup not
only the information of the pattern elements 22b but also the
information of the underlying substrate 22 such as the thickness of
a film forming a top part of the substrate 22. In such a case,
there is a substantial risk that the width W obtained from the
ellipsometric parameters .psi. and .DELTA. deviates from the real
value of the width W.
Thus, the present invention exploits the coherent nature of the
optical beam 23, which is a He-Ne laser, and obtains the pattern
width W from diffraction optical beams that are produced by the
line-and-space pattern 22a as a result of Bragg diffraction. In
this case, one or more photocells are disposed so as to detect one
or more of such diffraction optical beams. Thereby, the pattern
width W is obtained based upon the observed ellipsometric
parameters .psi. and .DELTA. similarly to the previous embodiment.
In the present embodiment, it is also possible to obtain the most
reliable value from the width W by taking a simple average or
weighted average of the width W obtained by various diffraction
beams.
FIG. 10 shows the examples of the parameters .psi. and .DELTA.
obtained form the diffraction optical beam according to the process
of FIG. 9 for a specimen in which a line-and-space resist pattern
having a height of 150 .mu.m is formed with a 5 .mu.m-pitch, in
comparison with the parameters .psi. and .DELTA. obtained from the
reflected beam for the same line-and-space pattern. In the example
of FIG. 10, it should be noted that the incident angle of the
optical beam is set to 77.degree. and hence the reflection beam is
formed with a reflection angle of 77.degree.. On the other hand,
the first-order diffraction beam is produced with a diffraction
angle of 58.degree..
FIG. 10 clearly indicates that the value of the parameters .psi.
and .DELTA. changes when the width W of the line-and-space pattern
is changed in the range between 110 nm-210 nm, wherein it should be
noted that the parameters .psi. and .DELTA. change respectively in
the range of 5-10.degree. and 30-60.degree. when the reflection
beam is used, while the parameters .psi. and .sub.-- change
respectively in the range between 60-85.degree. and
-100--110.degree. when a first-order diffraction optical beam is
used. The result of FIG. 10 indicates further that the change of
the phase difference .DELTA. is small when the first-order
diffraction beam is used. In such a case, the width W is generally
proportional to the parameter .psi..
THIRD EMBODIMENT
FIG. 11 is a block diagram showing the construction of a production
line of semiconductor devices according to a third embodiment of
the present invention that uses the ellipsometer of FIG. 5.
Referring to FIG. 11, the production line includes a wafer
processing part 101 that may include in turn processes such as an
exposure process and an etching process, and a control part 102
that controls the wafer processing part 101 as usual in a
semiconductor process, wherein the production line of FIG. 11
further includes an ellipsometry part 103 that examines the wafer
processed by the processing part 101.
The ellipsometry part 103 includes the ellipsometer of FIG. 5 and
obtains the ellipsometric parameters .psi. and .DELTA. based upon
the polarization of an optical beam reflected from the wafer that
has been processed by the processing part 101. The ellipsometric
parameters .psi. and .DELTA. thus obtained are then supplied to the
control part 102, wherein the control part changes the process
condition such as the exposure dose, exposure time, RF power, and
the like, based on the comparison of the observed parameters .psi.
and .DELTA. with corresponding present values of the parameters
.psi. and .DELTA. that are supplied from a presetting part 104.
As indicated in FIG. 11, the presetting part 104 is supplied with
pattern data such as the thickness and shape of the line-and-space
pattern formed in the wafer process part 101 or other various data
such as the thickness of the layer formed under the line-and-space
pattern, and calculates the expected values of the parameters .psi.
and .DELTA. be referring to a database 104a that holds the
parameters .psi. and .DELTA. similarly to FIG. 7. Thereby, the
control part 102 compares the parameters .psi. and .DELTA. obtained
from the database 104a with the parameters .psi. and .DELTA.
obtained by the ellipsometer 103 and controls the process in the
processing part 102 such that the difference between the parameters
.psi. and .DELTA. of the database 104a and the actually observed
parameters .psi. and .DELTA. is minimized.
In the production line of FIG. 11, it should be noted that the
database 104a is constructed by examining the wafers of which
pattern size is already known by a SEM scanning, by using the
ellipsometer 103 and by storing the ellipsometric parameters .psi.
and .DELTA. thus obtained as a function of the shape parameter such
as the width W.
Alternatively, the parameters .psi. and .DELTA. may be calculated
theoretically in the system of FIG. 11 by using a theoretical
calculation unit 104b, such that the theoretically obtained
parameters .psi. and .DELTA. are supplied to the process control
part 102 via the presetting part 104.
The theoretical calculation of the parameters .psi. and .DELTA. in
the calculation unit 104b is conducted generally as follows.
When an optical beam is incident to a specimen carrying thereon a
periodic line-and-space pattern with an incident angle
.theta..sub.1, the incident optical beam experiences refractions
and reflections according to the Snell's law as it passes through
the specimen as indicated in FIG. 12, and the optical beam exits
from the line-and-space pattern with the same angle .theta..sub.1.
In FIG. 12, it should be noted that the environment of the specimen
has a refractive index n.sub.1 while the line-and-space pattern has
a refractive index n.sub.2. Further, the substrate on which the
line-and-space pattern is formed, has a refractive index
n.sub.3.
Thus, the incident optical beam is divided into m incident rays
I.sub.(1), I.sub.(2), . . . , I.sub.(m) and the optical path is
calculated for each of the incident rays I.sub.(1), I.sub.(2), . .
. I.sub.(m) by applying the Snell's law. In such a calculation,
each of the rays is decomposed into a p-polarization component and
an s-polarization component and the effect of attenuation
associated with reflection and refraction is calculated for each of
the p- and s-components.
More specifically, the effect of attenuation associated with the
reflection of an incident ray is evaluated by multiplying Fresnel's
amplitude reflectance coefficients r.sub.p and r.sub.s to the
amplitude I.sub.0 of the p-component and s-component of the
incident ray respectively each time the incident ray experiences a
reflection. When the incident ray experiences a refraction, on the
other hand, Fresnel's amplitude refraction coefficients t.sub.p and
t.sub.s are multiplied respectively to the intensity I.sub.0 of the
p- and s-components of the incident ray each time the incident ray
experiences a refraction. Further, the effect of attenuation caused
by an opaque medium is evaluated by multiplying an amplitude
transmittance coefficient t.sub.k.
The final intensity Ip(n)' and Is(n)' respectively of the p- and s-
components are then obtained by further multiplying the effect of
phase retardation .delta. associated with the optical path length,
wherein the amplitude reflectance coefficients r.sub.p and r.sub.s,
the amplitude refraction coefficients t.sub.p and t.sub.s, the
amplitude transmittance coefficient t.sub.k, and the phase
retardation .delta. are given respectively as
wherein .lambda. represents the wavelength of the incident optical
beam while d represents the optical path length of the optical beam
in the specimen.
In the example of FIG. 13, incident rays (1)-(3) having an initial
intensity I.sub.0 form exiting rays having p-components
Ip(1)'-Ip(3)' and s-components Is(1)'-Is(3)' given as
wherein n.sub.2 and k.sub.2 represent the refractive index and
absorption coefficient of the line-and-space pattern, while d.sub.1
and d.sub.2 represent respectively the optical path lengths of the
rays (1) and (2) in the line-and-space pattern. Further, the
suffices 1-5 represent the point of reflection or refraction
counted from the side where the rays enter the line-and-space
pattern. See FIG. 13.
After the intensities I(1)'-I(n)' are thus obtained, the complex
reflection coefficient ratio (Rp/Rs) is obtained according to the
relationship
and the parameters are obtained according to the relationships
of
In the construction of FIG. 11, the calculation unit 104b carries
out the foregoing calculation and supplies the obtained parameters
.psi. and .DELTA. to the process control unit 102 via the
presetting unit 104 as noted already.
FIG. 14 is a flowchart showing the processing carried out by the
calculation unit 104b.
Referring to FIG. 14, the process starts with a step 1 in which
various structural data such as the thickness, refractive index and
absorption of the pattern, the thickness, refractive index and
absorption of the underlying layer, the pattern pitch, and the like
are given in the form of input.
Next, in a step 2, the incident optical beam is decomposed into
individual rays 1-n, and the optical path is obtained for each of
the rays thus decomposed. Further, the intensities Ip(i)' and
Is(i)' of the p- and s- components are obtained in a step 3 for
each of the rays i(=1-n).
Further, in the step 4, the intensities Ip(i)' and Is(i)' obtained
previously for the rays i are summarized respectively for all of
the rays 1-n, and integral intensities .SIGMA.Ip(n)' and
.SIGMA.Is(n)' are obtained. The integral intensities thus obtained
are then used in the step 5 to obtain the complex reflection
coefficient ratio .SIGMA.Ip(n)/.SIGMA.Is(n), and the complex
reflection coefficient ratio .SIGMA.Ip(n)/.SIGMA.Is(n) thus
obtained is used in a step 6 to calculate the parameters .psi. and
.DELTA.. Further, in a step 7, a reference is made to the database
for the specific combination of the parameters .psi. and .DELTA.
obtained in the step 6, and the value of the pattern W is obtained
as a result of such a reference of the database.
FIG. 15 shows the relationship between the parameter .psi. and
.DELTA. obtained by the ellipsometer of FIG. 5 for the substrate 22
described previously with reference to FIGS. 6A and 6B, for a state
in which an RIE (reactive ion etching) process is applied to the
polysilicon layer 22.sub.2 while using the resist pattern 22a as a
mask and the resist pattern 22a is removed subsequently. In the
example of FIG. 15, it should be noted that the incident angle of
the incident optical beam is set to 55.degree., not 70.degree.. By
accumulating the relationship of FIG. 15 for various patterns, the
database 104a is constructed.
FIG. 16, on the other hand, indicates the relationship of the
parameters .psi. and .DELTA. observed for the case in which the
line-and-space pitch of the mask pattern 22a is set to 150 nm in
the embodiment of FIGS. 6A and 6B. In FIG. 16, it should be noted
further that the duration of the etching process is changed
variously. Thus, FIG. 16 shows the results in which an excessive
etching is applied to the polysilicon layer 22.sub.2 by 10% and
30%, in addition to the case in which the etching is terminated
exactly upon the exposure of the underlying SiO.sub.2 layer
22.sub.1.
When such an excessive etching process is applied in such an RIE
process for forming the line-and-space pattern in the polysilicon
layer 22.sub.2, it is known that the line-and-space pattern thus
formed experiences a substantial side wall etching. For example,
when an excessive etching of 30% is made, it is known, from a SEM
observation, that the line-and-space pattern thus formed
experiences a side wall etching of as much as 20 nm. The result of
FIG. 16 clearly indicates that the present invention for applying
ellipsometry to line-and-space patterns for the measurement of the
pattern width W is effective also for detecting the side wall
etching of the pattern within a precision of about 10 nm.
FOURTH EMBODIMENT
FIG. 17 shows the principle of a fourth embodiment of the present
invention.
Referring to FIG. 17, the present embodiment obtains the foregoing
ellipsometric parameters .psi. and .DELTA. based upon the
scattering of an incident optical beam In caused by the pattern 22b
on the substrate 22, rather than using a reflection beam or
diffraction beam of the incident optical beam.
FIG. 18 shows the construction for carrying out the measurement of
a scattered light indicated in FIG. 17, wherein those parts
described previously are designated by the same reference numerals
and the description thereof will be omitted.
In the apparatus of FIG. 18, the coherent optical beam emitted from
the optical source 23 reaches the wafer 22 which carries thereon
the pattern 22a, after passing through the polarizer 25 and further
through a beam chopper 25A, wherein the beam chopper 25A is a
rotary disk formed with a cutout at a part thereof for turning on
and turning off the optical beam incident to the wafer 22 from the
optical source 23.
Further, the construction of FIG. 18 includes a lens 31 and a beam
splitter 32 cooperating with the lens 31, wherein the lens 31 is
disposed so as to avoid the reflection beam or diffraction beam
produced by the pattern 22a on the wafer 22. Thereby, the beam
splitter 32 decomposes the scattered light produced by the pattern
22a and focused by the lens 31 into the p-component and the
s-component and supplies the p- and s-components thus decomposed to
a detector 27A and a detector 27B respectively. The detectors 27A
and 27B are supplied with a control signal from the beam chopper
25A as a synchronizing signal SYNC and detect the intensities Ip
and Is of the scattered light respectively, wherein the control
signal is used in the beam chopper 25A for controlling the rotation
thereof and
hence the on-off control of the optical beam. According to such a
construction, it becomes possible to detect the intensities Ip and
Is of feeble scattering light with high precision, by comparing the
detected intensity with the background intensity in which the
incident optical beam is interrupted.
The intensities Ip and Is of the scattered light thus obtained is
then processed in the processing apparatus 30, wherein a
calculation is made in the processing apparatus 30 to divide the
intensity Ip by the intensity Is (Ip/Is), to obtain a reflection
coefficient ratio tan.psi. and hence the ellipsometric parameter
.psi..
FIG. 19 shows the relationship between the reflection coefficient
ratio (tan.psi.=Rp/Rs) and the pattern width W obtained according
to the construction of FIG. 18 that uses the scattering of the
incident optical beam. In the example of FIG. 19, it should be
noted that a wafer carrying a line-and-space pattern having a
thickness of 180 nm and a pattern pitch of 300 nm on a SiO.sub.2
film of a thickness of 100 nm is used as the substrate 22, wherein
the pattern width W is changed between 120 nm and 180 nm. The
ellipsometric measurement was made by using a linearly polarized
beam having a polarization angle of 45.degree. as the incident
optical beam.
As will be seen from FIG. 19, the reflectance coefficient ratio
tan.psi. clearly shows a dependency on the pattern width W. This in
turn means that it is possible to obtain the pattern width W of the
line-and-space pattern from the reflection coefficient ratio
tan.psi. of the scattered light.
FIFTH EMBODIMENT
In any of the ellipsometers of FIG. 5, FIG. 9, FIG. 11 or FIG. 17,
the incident angle of the optical beam emitted from the optical
source 23 is by no means limited to 70.degree. but other incident
angle may also be used. Further, by changing the incident angle
variously, it is possible to evaluate not only the pattern width W
but also the inclination angle of the pattern side wall by
referring to a database that includes the incident angle as a
parameter in addition to the foregoing ellipsometric parameters
.psi. and .DELTA..
FIG. 20 shows the ellipsometric parameters .psi. and .DELTA.
obtained from a reflection beam produced by a wafer in which a
polysilicon film is formed with a thickness of 182 nm on a
SiO.sub.2 film having a thickness of 102 nm.
Referring to FIG. 20, it should be noted that the parameters .psi.
and .DELTA. are obtained as indicated in the upper left curve of
FIG. 20 in the state in which the etching of the polysilicon film
is not started yet and only a line-and-space resist pattern is
formed on the polysilicon film with pattern widths W of 175 nm, 150
nm and 125 nm. In FIG. 20, it should be noted that three
measurements, represented by an open circle, an open triangle and
an open square, were made for each of the pattern widths W of 175
nm, 150 nm and 125 nm, wherein it is noted that the convergence of
these measurements are excellent. The data of FIG. 20 was taken by
setting the incident angle of the optical beam to 70.degree..
Next, an etching is applied to the foregoing polysilicon film wile
using the line-and-space resist pattern as a mask and the
ellipsometric measurement was made upon the structure thus
obtained, wherein the solid triangles represent the parameters
.psi. and .DELTA. obtained for the structure in which the etching
is stopped immediately upon exposure of the underlying SiO.sub.2
film (0% prolonged etching). On the other hand, the solid squares
represent the parameters .psi. and .DELTA. obtained for the
structure in which the etching is continued for a duration of 30%
the nominal duration of etching (30% prolonged etching), after the
underlying SiO.sub.2 film is exposed. Further, the solid circles
represent the parameters .psi. and .DELTA. for the structure in
which the etching is continued for a duration of 80% the nominal
duration of etching (80% prolonged etching), after underlying
SiO.sub.2 film is exposed.
The structure corresponding to the solid triangles is represented
in FIG. 21A for the case in which the pattern width W is 150 nm,
the structure corresponding to the solid squares is represented in
FIG. 21B for the case in which the pattern width W is also 150 nm,
and the structure corresponding to the solid circles is represented
in FIG. 21C for the case in which the pattern width W is again 150
nm, wherein FIGS. 21A-21C show the cross-section of the polysilicon
line-and-space patterns obtained by a SEM observation.
As will be seen clearly from FIG. 20, the curves for the various
prolonged etchings are different from each other, except for the
case where the pattern width W is set to 175 nm. Only in this case,
the curves overlap partially. This means that it is generally
possible to evaluate also the cross sectional shape of the
polysilicon patterns as represented in FIGS. 21A-21C, from the
combination of the ellipsometric parameters .psi. and .DELTA., when
it is possible to estimate the structural parameters of the
line-and-space pattern such as the pattern width W, film thickness,
refractive index, and the like, by some other means.
SIXTH EMBODIMENT
FIG. 22 shows the relationship between the ellipsometric parameters
.psi. and .sub.-- of the same polysilicon line-and-space pattern
for the case in which the thickness of the polysilicon film is
changed variously on the SiO.sub.2 film of which thickness is set
to 100 nm, wherein the curve represented in FIG. 22 by solid
circles represent the case in which the polysilicon line-and-space
pattern has a thickness of 178 nm, while the curve represented by
open circles represent the case in which the polysilicon
line-and-space pattern has a thickness of 163 nm. In any of these
cases, the line-and-space pattern was formed to have a pitch of 300
nm, and the width W is changed in the range between 110-200 nm.
As will be seen clearly in FIG. 22, the curve for the 178 nm
thickness is distinctly different from the curve for the 163 nm
thickness. This in turn means that it is possible to apply the
ellipsometric process of the present invention for the patterns of
various thicknesses by measuring the pattern thickness before
proceeding to the ellipsometric process.
FIG. 23 shows the flowchart of such a process that includes the
step of measuring the film thickness according to a sixth
embodiment of the present invention.
Referring to FIG. 23, a step 11 is conducted at first for
determining the thickness of the line-and-space pattern, and a step
12 is conducted subsequently for selecting the characteristic curve
for the measured thickness of the line-and-space pattern. Further,
an ellipsometric process is conducted in a step 13 for the
parameters .psi. and .DELTA., and the pattern width W or the cross
sectional shape thereof is obtained based upon the characteristic
curve thus selected.
Further, the present invention is not limited to the embodiments
described heretofore, but various variations and modifications may
be made without departing from the scope of the invention.
* * * * *